If you've put some effort and a scholly on an Undecided who still is undecided, and no other human puts any effort into him, can he go to considering and then sign last cycle? Or will he likely just disappear into the ether, never to be heard from again?

I believe he needs to be green to sign. Toward the end, you might be able to get him green with just a phone call, but he needs to be green, and you need at least one face-to-face meeting (e.g., AC scout). Hope this helps!